Fix Solid, SolidStart, and NestJS README snippet errors

[nicohrubec]

Fix copy/paste errors in three package README examples so they use the correct imports.

Solid and SolidStart

In packages/solid/README.md and packages/solidstart/README.md, the examples import withSentryRouterRouting from the dedicated /solidrouter entry point but call it on the main Sentry namespace, which does not export it. Use the function already imported:

- const SentryRouter = Sentry.withSentryRouterRouting(Router);
+ const SentryRouter = withSentryRouterRouting(Router);

NestJS

The cron example in packages/nestjs/README.md imports MonitorConfig twice, causing a duplicate identifier. Keep the type-only import from @sentry/core:

- import { SentryCron, MonitorConfig } from '@sentry/nestjs';
+ import { SentryCron } from '@sentry/nestjs';
  import type { MonitorConfig } from '@sentry/core';

[linear-code]

JS-3735